This invention relates to semiconductor fabrication. More particularly, it relates to an improved apparatus for the treatment of semiconductor wafers with fluids.
In the process of fabricating semiconductor wafers, the need to avoid contamination by impurities takes on critical importance. Many of the steps require that the wafer in process be exposed to a fluid, e.g., an oxidizing agent, etchant, or a washing or rinsing agent. To achieve acceptable semiconductor yields, it is important that each of these steps be carried out so as to minimize the potential for contamination.
For example, the diffusion step in semiconductor processing is intended to "drive-in" desired dopant atoms. Any contaminants left on the wafer surface after pre-diffusion cleaning may also be driven into the wafer and can result in a product having faulty electrical properties.
Pre-diffusion cleaning conventionally is accomplished by loading the wafers into a "basket" or wafer carrier such as those described in U.S. Pat. Nos. 3,923,156 and 3,926,305, and then immersing the wafers in the basket in a series of open sinks or tank baths. Usually, the cleaning method involves contacting the wafers first with a strong oxidizing agent, such as sulfuric acid, rinsing with water, contacting the wafers with an etchant such as hydrofluoric acid, and then rinsing again.
Conventional open bath processing systems pose a number of problems. For example, in moving the wafers from the etchant tank to the dump-rinser tank, drops of etchant adhere to the wafers, and during the brief but significant time between tanks, localized etching can occur. With the micron-size dimensions typical of semiconductor fabrication, such minor discrepancies can have a major impact on the yield and performance of integrated circuits.
The so-called "clean rooms" in which open bath processing is performed contain a variety of contaminant sources. Contaminants can originate from human contact, solvent residues and oils from various operations, metals, dust particles, organics and other materials present in processing reagents and in air. Impurities arising from these sources can and do contaminate the reagents and rinse water, leading to reduced semiconductor yields. Further, open baths mounted in countertops pose significant safety problems to technicians who may be exposed to active reagents or their fumes.
The above-mentioned co-pending application Ser. No. 612,355 discloses a substantial improvement in the art of semiconductor wafer processing. That application discloses an enclosed treatment system in which wafers are placed in a vessel which is connected as an integral part of a treatment fluid flow line. The system permits efficient pre-diffusion cleaning and reduces the risk of both wafer contamination and worker exposure to harmful processing fluids.
In the disclosed flow line treatment system of co-pending application, Ser. No. 612,355, liquids entering the vessel through the inlet element to displace another tend to flow directly into the central, wafer-containing volume with unpredictable swirling currents. Each wafer can act as a baffle by at least temporarily inhibiting an adjacent volume of processing fluid from contacting other wafers. Consequently, the innermost wafers can receive greater exposure with a resulting small but significant nonuniformity in the extent of treatment of wafers within a batch, and lack of repoducibility from batch to batch.
It is an object of this invention to provide an improved semiconductor wafer treatment system. More particularly, an object of the invention is to provide an enclosed wafer processing system which reduces risk of contaminant introduction and worker injury, while, simultaneously, providing uniform, reproducible exposure of wafers to various treatment fluids. Another object is to provide an enclosed semiconductor wafer processing system in which treatment fluids are introduced into the vessel in a manner which improves yield.
Other objects of the invention will be evident from the description below.